In industrialized societies it has become commonplace for spills of hazardous materials to occur. Sometimes these spills occur at sea due to, for example, a leak in an oil tanker. Various technologies have been developed to deal with this situation.
More often, spills occur on land or even underneath the ground. For example, underground gasoline tanks may leak or solvents used in industrial processes may be illegally discharged into waste water or sumps or directly onto the ground. In any case, the hazardous material, typically a volatile organic compound, may propogate great distances through the ground and even enter ground water aquifers. The resulting environmental impact may be devastating.
Various vacuum techniques have been applied to the problem of extracting these volatile hazardous materials from the ground. In a first, primitive technique, a rather long and deep ditch is dug in the ground where contamination has occurred. A pipe with a multitude of small, screened openings is then placed horizontally at the bottom of the ditch. This pipe is then connected to a vertical pipe which extends along a wall of the ditch to the surface. A vacuum blower located on the surface completes this extraction site.
To use this method, it is necessary to dig up the soil at the spill site that has been contaminated. The contaminated soil is then transported to the extraction site where it is used to fill in the ditch. A vacuum is then pulled on the vertically-extending pipe using the vacuum blower. Eventually, assuming the hazardous material is at all volatile, it is removed from the soil. The soil is then removed from the ditch and used as land fill at either the original spill site or a new location.
This technique suffers from several major disadvantages. First, it is necessary to disrupt the spill site and remove the soil, at least on a temporary basis. Further, it is exceedingly expensive to cart the material to the extraction site and back to the spill site or another land-fill site. Finally, as more specifically outlined below, there are some environmental objections to this method of removal which, if it is to be useful, require an extremely low hazardous material removal rate.
Another approach used to remove such hazardous materials from the ground does not require that the soil be removed and transported. Instead, a shallow vertical well is drilled. Another shallow vertical well is then drilled a short distance from and substantially parallel to the first well. A vacuum blower pulls air from the first well while air is permitted to enter the second well. Each well may have in it a snugly fitting pipe having numerous, preferably screened, openings therein along its entire length in a manner similar to the pipe used in the first technique described above.
While this technique is more economically efficient because the transportation of soil is unnecessary, it also suffers from several major disadvantages. First, since there are openings along the entire length of the pipe, the vacuum is drawn on areas of the soil which are permeable to air and on areas that are relatively impermeable. The permeable areas tend to act as shunt passages through which large quantities of air are drawn, thus detracting from the amount of air drawn through less permeable areas. However, volumes of soil which are less permeable to air may nevertheless be heavily impregnated with the hazardous solvents. These areas are not purged of hazardous material. The hazardous material thus remains in the ground and continues to be a potential source of an environmental disaster. Another major disadvantage of this technique is that only a limited volume of ground soil is treated. While this disadvantage may be overcome by forming, in addition to the primary well, a plurality of secondary wells surrounding the primary well (preferably in a circle), this produces another significant problem.
When hazardous materials are removed from the ground by this technique, air containing the vapors of the organic solvents is discharged into the atmosphere. There are federal, state and local environmental regulations which place a limit on the amount of solvent that may be discharged in this manner. For example, a typical limit is 3.5 pounds (1.6 kilograms) per hour for volatile organic compounds. If a deep well is drilled, it is not uncommon for one well alone to produce 2.5 pounds of solvent per hour discharged into the air. Thus, it is impossible to uncap several of the secondary wells simultaneously to increase the speed at which the solvent is removed from the ground because permissible environmental ambient air quality limitations would be exceeded. In the case of a large spill, it could take years to effect a proper clean up.
There are techniques which allow for increasing the speed of the clean up. The most common approach is to use an activated charcoal filter to adsorb the vapors of the hazardous material. While placing such a filter in the atmospheric discharge conduit of the vacuum blower is a relatively simple procedure, the cost is quite large. The filter has only a finite life, after which it must be taken to a processing plant to be stripped of the hazardous material. Unless a replacement filter is immediately available, this disrupts the purging process. Further, with the use of spare filters, there is a practical financial limit to the speed at which the ground can be purged. In the limit, this is due to the cost of reactivating the adsorbent (e.g. charcoal) for reuse.